Oh man I'm going to ruffle some feathers here - but I"m writing this as much for me as anyone else so here goes...
Nothing drives me more crazy than when I hear someone (usually a perfectly health person say) - "I can't do that."
Ok - I know you can't. It's hard right? I got it.
But the "I can't" statement is a door slammer. It shuts out any possibility that you may be able to do the pose either in the immediate or near future.
Looking further at this mindset you see what is really true. What's true is the "I can't" statement is the ego's smokescreen. Because truly the problem isn't that the body "can't" do the pose. The poses are MADE for the body. The body LOVES the poses.
The problem is always the mind - and the mind lies to us. The truth is either one (or all) of the following....
"I can't do" the pose because...
...I dont' practice
...I'm scared it will hurt
...I don't want to work that hard
...I don't like looking silly trying
...I just don't want to do it.
At least these have all been (and continue to be) true for me to varing degrees.
It's always the mind that keeps us from practicing the pose. The mind stops, stalls, resists, argues, complains, whines and basically keeps us stuck. Furthermore, the mind talks us into believing the crap that it tells us...that my body can't do that. Bully-pucky!
That's why the classic Yoga Sutra (1:2) is - "Yogah chitta vttri nirodah"
Yoga is the process of extinguishing the "chatter" of the mind - the vttri's - literally the "twirlings" of the mind.
Next time you practice - when you hear your self about to say "I can't" - stop it! Don't listen - just breath and feel - let the mind observe but NEVER lead the practice. The practice should always be led by the Guru within. The inner-being - the Atman - who resides in your heart.